Conventionally, several kinds of variable valve timing devices (VVT) are known in this field. For example, a vane type VVT has a housing and a vane rotor relatively rotatable to the housing. The housing is adapted to be rotated with one of a crankshaft and a cam shaft. The vane rotor is housed in the housing and is adapted to be rotated with the other one of the crankshaft and the cam shaft. JP2010-163942A discloses a device that varies a relative rotational phase of the vane rotor with respect to the housing in an advancing direction or a retarding direction. The device varies a phase difference by controlling fluid flow, e.g., by introducing operational fluid into an advancing chamber or a retarding chamber both defined within the housing by dividing a housing chamber by the vane rotor. The VVT may include a control valve having a sleeve and a spool which is slidably supported and housed in the sleeve in a reciprocal manner. The control valve switches and controls introducing flow and discharging flow to the advancing chamber and the retarding chamber in accordance with an axial position of the spool.
The axial position of the spool is controlled in accordance with a balance between an axial driving force and an axial biasing force. The driving force is generated by a driving source in response to an instruction value. The biasing force urges the spool in an opposite direction to the driving force, and is adjusted by a device which provides bias adjusting means, i.e., bias setting means. The bias adjusting means varies the biasing force in a step like manner at a boundary position between two regions. The regions correspond to a movable range of the spool and are defined adjacent to each other in an axial direction. By generating such a step like characteristic on the biasing force by the bias adjusting means, the control valve demonstrates required performance. For this purpose, the control valve may include a movable member and a pair of resilient members. The movable member and the resilient members may be housed in the sleeve.
In the first region, the movable member engages with the spool with respect to the axial direction, and moves with the spool. On the other hand, in the second region, the movable member abuts and rests on the sleeve with respect to the axial direction, and enables a relative movement of the spool. The first resilient member generates a first restoring force that axially urges the spool in both the first region and the second region. The second resilient member generates a second restoring force that axially urges the movable member in both the first region and the second region. By employing such arrangement, since the spool engaged with the movable member by the second restoring force is urged in the axial direction in the first region, a summed force of the second restoring force and the first restoring force acts as a biasing force against the driving force. On the other hand, in the second region, since a movable member is abut on the sleeve and an urging of the spool by the second restoring force is restricted, only the first restoring force acts as the biasing force against the driving force. Accordingly, the urging force may be changed in a step like manner. This is caused by switching acting force at a boundary position between the first and the second region. The acting force is switched between the summed force of the second restoring force and the first restoring force, and a single force of the first restoring force.